Intrigued by the prospect of scripting GIMP from the commandline I decided to see if I could figure out how to apply filters in a batch to a whole bunch of images. And I did.
And the results are terrifying.
Code and Things
So Gimp uses a Scheme scripting language called ScriptFu. It's a quite a bit different from Python and a bit lower level. The script I got working is based on this tutorial.
There were some issues with transparency and stuff so it took a bit of experimenting to get it to work. And I don't understand ScriptFu's syntax very well, so there's that. Anyway, here's the code:
(define (bulk-filter pattern)
(let* ((filelist (cadr (file-glob pattern 1))))
(while (not (null? filelist))
(let* ((filename (car filelist))
(image (car (gimp-file-load RUN-NONINTERACTIVE filename filename)))
(drawable (car (gimp-image-get-active-layer image))))
(plug-in-glasstile 1 image drawable 20 20)
(set! drawable (car (gimp-image-get-active-drawable image)))
(gimp-file-save RUN-NONINTERACTIVE image drawable filename filename)
(gimp-image-delete image))
(set! filelist (cdr filelist)))))
Save it as scriptname.scm in ~/.gimp-2.8/scripts. The file name doesn't really matter.
To run the script, navigate into the folder you want to effect in the terminal then run the following command:
$ gimp -i -b "(bulk-filter \"*.png\")" -b "(gimp-quit 0)"
Be sure your images are backed up somewhere (or, like we are doing, in a git repo that can be git reset --hard when things are ugly).
Modifying the script
The line of code that does the magic is:
(plug-in-glasstile 1 image drawable 20 20)
You can modify it to take almost any GIMP filter (though bugs may result). You can find the code for more filters by running the Script Fu console in GIMP:
Clicking on Browse
Searching for the name of a filter
Note on the right hand side of the window the parameters.The best way to figure out what the variables should be is to run the filter in GIMP on a trial image and see what variable values it suggests.
For reference INT32 parameters are Boolean values (1 or 0) and FLOAT are numbers. Image and Drawable are already set by the script.
So to use the cubism plugin I change the code to say:
(plug-in-cubism 1 image drawable 10 2.5 1)
Producing:
If you want to do more complicated batch file processing in GIMP I would recommend the BIMP Batch Image Manipulation Plugin. It does all of the heavy lifting for you and isn't nearly as finicky.